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BZERO(3) Linux Programmer's Manual BZERO(3)
bzero, explicit_bzero - zero a byte string
#include <strings.h>
void bzero(void *s, size_t n);
#include <string.h>
void explicit_bzero(void *s, size_t n);
The bzero() function erases the data in the n bytes of the memory
starting at the location pointed to by s, by writing zeros (bytes
containing '\0') to that area.
The explicit_bzero() function performs the same task as bzero().
It differs from bzero() in that it guarantees that compiler
optimizations will not remove the erase operation if the compiler
deduces that the operation is "unnecessary".
None.
explicit_bzero() first appeared in glibc 2.25.
For an explanation of the terms used in this section, see
attributes(7).
┌─────────────────┬───────────────┬─────────┐
│Interface │ Attribute │ Value │
├─────────────────┼───────────────┼─────────┤
│bzero(), │ Thread safety │ MT-Safe │
│explicit_bzero() │ │ │
└─────────────────┴───────────────┴─────────┘
The bzero() function is deprecated (marked as LEGACY in
POSIX.1-2001); use memset(3) in new programs. POSIX.1-2008
removes the specification of bzero(). The bzero() function first
appeared in 4.3BSD.
The explicit_bzero() function is a nonstandard extension that is
also present on some of the BSDs. Some other implementations
have a similar function, such as memset_explicit() or memset_s().
The explicit_bzero() function addresses a problem that security-
conscious applications may run into when using bzero(): if the
compiler can deduce that the location to zeroed will never again
be touched by a correct program, then it may remove the bzero()
call altogether. This is a problem if the intent of the bzero()
call was to erase sensitive data (e.g., passwords) to prevent the
possibility that the data was leaked by an incorrect or
compromised program. Calls to explicit_bzero() are never
optimized away by the compiler.
The explicit_bzero() function does not solve all problems
associated with erasing sensitive data:
1. The explicit_bzero() function does not guarantee that
sensitive data is completely erased from memory. (The same is
true of bzero().) For example, there may be copies of the
sensitive data in a register and in "scratch" stack areas.
The explicit_bzero() function is not aware of these copies,
and can't erase them.
2. In some circumstances, explicit_bzero() can decrease security.
If the compiler determined that the variable containing the
sensitive data could be optimized to be stored in a register
(because it is small enough to fit in a register, and no
operation other than the explicit_bzero() call would need to
take the address of the variable), then the explicit_bzero()
call will force the data to be copied from the register to a
location in RAM that is then immediately erased (while the
copy in the register remains unaffected). The problem here is
that data in RAM is more likely to be exposed by a bug than
data in a register, and thus the explicit_bzero() call creates
a brief time window where the sensitive data is more
vulnerable than it would otherwise have been if no attempt had
been made to erase the data.
Note that declaring the sensitive variable with the volatile
qualifier does not eliminate the above problems. Indeed, it will
make them worse, since, for example, it may force a variable that
would otherwise have been optimized into a register to instead be
maintained in (more vulnerable) RAM for its entire lifetime.
Notwithstanding the above details, for security-conscious
applications, using explicit_bzero() is generally preferable to
not using it. The developers of explicit_bzero() anticipate that
future compilers will recognize calls to explicit_bzero() and
take steps to ensure that all copies of the sensitive data are
erased, including copies in registers or in "scratch" stack
areas.
bstring(3), memset(3), swab(3)
This page is part of release 5.10 of the Linux man-pages project.
A description of the project, information about reporting bugs,
and the latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2019-03-06 BZERO(3)
Pages that refer to this page: bstring(3), memset(3)
Copyright and license for this manual page
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